Steel parts used in automobiles and various industrial machinery etc. are improved in fatigue strength, wear resistance, seizing resistance, and other mechanical properties by carburizing hardening, high-frequency hardening, nitriding, soft nitriding, and other surface hardening heat treatment.
Nitriding and soft nitriding are performed in the ferrite region of the A1 point or less. During treatment, there is no phase transformation, so it is possible to reduce the heat treatment strain. For this reason, nitriding and soft nitriding are often used for parts requiring high dimensional precision and large sized parts. For example, they are applied to the gears used for transmission parts in automobiles and the crankshafts used for engines.
Nitriding is a method of treatment diffusing nitrogen into the surface of a steel material. For the medium used for the nitriding, there are a gas, salt bath, plasma, etc. For the transmission parts of an automobile, gas nitriding is mainly being used since it is excellent in productivity. Due to gas nitriding, the surface of the steel material is formed with a compound layer of a thickness of 10 μm or more. Furthermore, the surface layer of a steel material at the lower side of the compound layer is formed with a nitrogen diffused layer forming a hardened layer. The compound layer is mainly comprised of Fe2-3N and Fe4N. The hardness of the compound layer is extremely high compared with the steel of the base material. For this reason, the compound layer improves the wear resistance and pitting resistance of a steel part in the initial stage of use.
However, a compound layer is low in toughness and low in deformability, so sometimes the compound layer and the base layer peel apart at their interface during use and the strength of the part falls. For this reason, it is difficult to use a gas nitrided part as a part subjected to impact stress and large bending stress.
Therefore, for use as a part subjected to impact stress and large bending stress, reduction of the thickness of the compound layer and, furthermore, elimination of the compound layer are sought. In this regard, it is known that the thickness of the compound layer can be controlled by the treatment temperature of the nitriding and the nitriding potential KN found from the NH3 partial pressure and H2 partial pressure by the following formula:KN=(NH3 partial pressure)/[(H2 partial pressure)3/2]
If lowering the nitriding potential KN, it is also possible to make the compound layer thinner and even eliminate the compound layer. However, if lowering the nitriding potential KN, it becomes hard for nitrogen to diffuse into the steel. In this case, the hardness of the hardened layer becomes lower and the depth becomes shallower. As a result, the nitrided part falls in fatigue strength, wear resistance, and seizing resistance. To deal with such a drop in performance, there is the method of mechanically polishing or shot blasting etc. the nitride part after gas nitriding to remove the compound layer. However, with this method, the production costs become higher.
PLT 1 proposes the method of dealing with such a problem by controlling the atmosphere of the gas nitriding by a nitriding parameter KN=(NH3 partial pressure)/[(H2 partial pressure)1/2] different from the nitriding potential and reducing the variation in depth of the hardened layer.
PLT 2 proposes a gas nitriding method enabling formation of a hardened layer (nitrided layer) without forming a compound layer. The method of PLT 2 first removes the oxide film of a part by fluoride treatment then nitrides the part. A non-nitriding material is necessary as a fixture for placing the treated part in a treatment furnace.
However, the nitriding parameter proposed in PLT 1 may be useful for control of the depth of the hardened layer, but does not improve the functions of a part.
As proposed in PLT 2, in the case of the method of preparing a non-nitriding fixture and first performing fluoride treatment, the problems arise of the selection of the fixture and the increase in the number of work steps.